Plasma deposition is the name given to any of a wide variety of processes in which a plasma is used to assist in the deposition of thin films or coatings onto the surfaces of objects. For example, plasma deposition processes are widely used in the electronics industry to fabricate integrated circuits and other electronic devices, as well as to fabricate the magnetic tapes and disks used in audio, video, and computer applications. Such plasma deposition processes may also be used in reverse, i.e., to remove material from the surfaces of objects, in which case they are usually referred to as plasma etching or plasma cleaning processes.
Regardless of whether the plasma process is used for deposition or etching, the plasma is usually created by subjecting a low-pressure process gas (e.g., argon) contained within a vacuum chamber to an electric field created between two electrodes. The electric field ionizes the process gas, creating the plasma. In the case of a sputter deposition plasma process, the target is usually connected as the negative electrode (i.e., the cathode). The ionized process gas atoms comprising the plasma are positively charged and are accelerated toward the negatively charged target/cathode. When the ions impact the surface of the target material they dislodge atoms of the target material, releasing them into the vacuum chamber. The substrate is usually positioned with respect to the target so that a majority of the dislodged (i.e., sputtered) atoms from the target material deposit themselves onto the surface of the substrate.
In the case where the target comprises an electrically conductive material, it is generally preferable to generate the plasma by using a direct current (i.e., DC). In this case, the target material is connected as the cathode whereas the other electrode (usually the vacuum chamber itself) is connected as the anode. Unfortunately, however, direct current (DC) generally cannot be used to generate a plasma if the sputtering target, substrate, or deposited film comprises a non-conductive material. Such non-conductive materials tend to acquire a repellant charge which eventually increases to the point where it inhibits the glow discharge and prevents sputtering action.
The charge accumulation problem associated with the use of non-conductive materials can be overcome by using an alternating current (i.e., AC) power source to generate the plasma. When an AC power source is used, current flows through the plasma until the insulating material (e.g., the target) accumulates a charge sufficient to terminate the glow discharge. Then, on the next half cycle, the accumulated charge is removed and current flows in the opposite direction until the insulating material again acquires a repellant charge, this time in the opposite sense. If the AC frequency is increased to the point where the period is much shorter than the time it takes the insulating material to acquire the repellent charge, then current will flow in the plasma during the entire AC cycle. Generally speaking, a frequency in the range of 50 kHz to 100 kHz is sufficient to achieve this condition.
While many different types of AC plasma processes are known and have been used for years, they continue to be plagued by the periodic occurrence of electrical discharges or arcs within the vacuum chamber. Such electrical discharges or arcs can take on different forms depending on the characteristics of the sputtering apparatus and on the particular plasma process being used. For example, arcs may occur between the target material and the substrate itself, certainly causing a defect in the coating, if not ruining the substrate entirely. Alternatively, the arc may occur between the target and some other part of the vacuum chamber, in which case the deleterious effects of the arc are usually less severe, but nevertheless tend to degrade the overall quality of the coating. The arcs can be short lived, lasting only a few milliseconds or so, or may be continuous, again depending on the particular apparatus and process being used.
Several methods for reducing the occurrence of such arc discharges rely on the selective control of the AC power supply used to place the charge on the electrodes. For example, one such method has been to simply turn-off the AC power supply as soon as an arc is detected, then turn it back on again once the arc has dissipated. While this method can effectively quench sustained arcs, the stored energy in most power supplies takes many cycles to dissipate, increasing the response time, i.e., the time it takes to completely remove the electrical potential from the electrodes, to the point where such devices cannot effectively quench short duration arc events. Consequently, all that is really accomplished is a reduction in overall deposition rate, with little or no reduction in the adverse effects produced by the arc event itself.
Another control method has been to momentarily interrupt (e.g., disconnect) the AC power supply from the electrodes during the arc event. While the response time of this method is usually considerably faster, i.e., the voltage can be removed from the electrodes within a few milliseconds or so, it is difficult to dissipate the stored energy in the power supply. Consequently, such methods tend to stress the power supply or switching devices used to disconnect the power supply to the point of burn-out.
While other devices exist and are being used with some degree of success, none has proven to be a panacea. For example, many such other devices can only effectively suppress certain types of arc events or only arcs created under certain conditions. Other devices may have more effective arc suppression characteristics, but are usually plagued with complex electronic circuits and devices, which may be expensive to produce and/or prone to failure.
Consequently, a need exists for a method and apparatus for preventing and/or suppressing arc events in AC plasma processes and under various operating conditions. Such a method and apparatus should allow for the effective suppression of arcs under a wide range of conditions, but without the need to resort to expensive or complex circuit elements.